It is well known in vehicles to have seat belts attached directly to the vehicle and not to the seat. This has some advantages, such as the vehicle provides a solid anchor for the seat belt and therefore good resistance to application of a load to the seat belt. There are some disadvantages, however. In order to allow for adjustable reclining of the seat, or a change in position of the seat relative to the vehicle a seat belt which is attached to the vehicle will need to be readily adjustable. Because the position of a seat belt is fixed relative to the vehicle, in some adjusted positions of the seat belt the seat may not be in the ideal position for the user. In general, integrated seat belts (where the seat belts are attached to the seat and not to the vehicle) are considered to be safer than seat belts that are anchored to the vehicle. Further, in some cases it is difficult to anchor a seat belt to a vehicle and the seat belt must be integrated with the seat. Examples of this include public transport eg. seats on buses that are not near the walls of the vehicles. In suspension seats, where the seat is suspended relative to the vehicle for comfort, an integrated seat belt is a safer and more convenient option. Suspension seats are often used in large vehicles such as trucks and military vehicles. Further, in some cases, legislation requires that seats must have integrated seat belts.
Where a seat is provided with an integrated seat belt, the seat must be arranged to cope with the very high loads that may be applied in the event of an accident. With lap-sash type or harness type seat belts, where the belt is anchored to an upper portion of the seat back, the seat must be especially strong to resist collapsing when load is applied by the sash or harness. It is in fact often the case that for some types of vehicles where particularly large loads may be applied, only lap belts are provided.
In order to deal with the high loads applied to some seats, it is known to utilise tether straps which attach the seat frame to the vehicle. In this way, at least some of the load applied by the integrated seatbelt can be transmitted via the seat frame to the vehicle. In the event of an accident, the tether strap prevents the seat from collapsing or moving under load. A problem with tether straps is that they are of a fixed length and require adjustment whenever the position of the seat is adjusted or the seat height is changed. If the tether strap is not readjusted correctly. This can be dangerous.